The invention relates to an arrangement and a method for the damping of vibrations during microscopic examinations of inorganic and organic material specimens at low temperatures. It has been found that fluorescence effects can be amplified at low temperatures by avoiding so-called quenching effects so that the need to add contrast enhancing fluorescent dyes to the specimen can in part be obviated.
Avoiding partly toxic fluorescent dyes is advantageous in organic specimens, since the analysis results can be distorted by such dyes. It is also advantageous for such studies to be able to use commercially available laser microscopes, which are standard equipment in many laboratories and are not specifically designed for low-temperature examinations.
The use of these microscopes requires placing a special measuring cell with sufficiently low overall level, in which the actual specimen is located, between the microscope lens and microscope stage in the optical path of the microscope. The measuring cell should also be couplable with the coordinates of the microscope stage drive and easily removable.
Electro-mechanical cryocoolers have proven effective for cooling the specimens to temperatures below 100K, sometimes down to 4K, since their closed helium circuits avoid costly replenishment with helium. An unavoidable disadvantage of electro-mechanical cryocooler is vibrations that are introduced by the compression of the helium and pumps for producing a vacuum for thermal insulation. Vibrations of less than 10 nm are desirable for microscopic examination in order to avoid loss of optical resolution.
Electro-mechanical cryocoolers with reduced vibrations are known, for example pulse tube coolers. However, due to the operating principle, the moving piston of the compressor also introduces here vibrations. An attenuation of 60 to 80 dB is required for the vibrations of the cold head of the cryocooler with respect to the oscillation amplitude of the specimen holder. To make matters worse, the housing of the cryocooler also generates vibrations that propagate through the fasteners to the instrument table and via the housing of the measuring cell to the microscope stand and the microscope stage. Since other sensitive instruments may be positioned on the instrument table, the introduction of vibrations to the instrument table is undesirable.
Three nested damping systems are therefore required that, on the one hand, relate to conducting the cold temperature to the specimen in a measuring cell through an insulated vacuum line and, on the other hand, to keeping vibrations away from the instrument table and the microscope.
Cryocoolers usually vibrate at a fundamental frequency of 1 . . . 5 Hz due to the movement of the compressor piston, wherein vibrations in the order of typically in the range of 50 Hz to 100 Hz occur at both end stops of the piston movement upon contact with the compressor housing, depending on the design of the end stop damper in the cryocooler. An optimum damping system should therefore be able to suppress both the movement frequency of the compressor piston and the vibration frequency resulting from the end stop as well as vibrations of the gas column.
In connection with the use of a mechanical cryocooler, for example, for electron microscopy, different arrangements are known for reducing the effect from vibrations, which affect both the cold conductor and the housing.
According to the prior art, the cold conductor located between the cold head of the cryocooler and the specimen can be decoupled, for example, by way of flexible copper strands (U.S. Pat. No. 4,161,747) or gas springs containing a cold-transmitting gas. These strands can be designed to be very compliant with correspondingly thin diameters of the individual wires, thus largely suppressing the transmission of vibrations.
It becomes much more difficult to reduce the coupling of the energetic housing vibrations of the cryocooler to the instrument table and the microscope. In the document U.S. Pat. No. 4,745,761, these vibrations relative to the base plate are damped by elastomer mats, which are compressed by screwing the housing to the base plate. The transmission of vibrations to a gamma ray detector is reduced by an elastic bellow disposed between the housing of the cryocooler and a mounting plate for the detector, wherein the mounting plate is also connected via elastic elastomer mats to the base plate. The cold head is decoupled by strands with respect to the transmission of vibrations.
However, the solution proposed in the document U.S. Pat. No. 4,745,761 for vibration damping of a gamma ray detector only be applied in a limited way to a measuring cell for microscopy according to the object of the present invention, since the mounting plate for the detector can perform natural oscillations due to the elastic connection to the base plate. In addition, the elastomer mats provided for damping are compressed by the ambient pressure and lose their elastic properties.
Another type of vibration isolation between the cryocooler and a receptacle is shown in U.S. Pat. No. 4,394,819. The receptacle is disposed in a measuring cell that is permanently connected to a base plate. The cold is transferred from a cold head of the receptacle by a flexible connection. The cryocooler is floatingly suspended between two springs designed as bellows. The bellows fulfill a dual function. On the one hand, the bellows function as a spring. On the other hand, the bellows enclose an evacuated chamber, which is used for thermal insulation of the cold conductor. The static forces, which are caused by the ambient pressure and compress the chambers, cancel each other in the selected arrangement. The vibration of the cryocooler is affected by damping elements which act between the base plate and the housing of the cryocooler. Although the damping elements reduce the vibration of the cryocooler housing, a portion of the kinetic energy causes opposing forces and vibrations of the base plate.
The mass of the cryocooler, together with the elastic properties of the bellows, forms a damped spring/mass system with a distinct natural frequency. The damping properties of a resilient suspension of a vibrating body can be improved by arranging a plurality of resilient regions with different elastic and damping characteristics in series, as described in the document AT 001 975. The damping elements are here arranged parallel to the springs and act in terms of their forces also directly on the base plate.
Active vibration damping systems are also known. The document U.S. Pat. No. 5,582,013 proposes to apply to the cryocooler a force that is phase shifted by 180° with respect to the vibrational forces of the cryocooler and has the same magnitude. These opposing forces are generated by a drive of a counterweight that can move relative to the cryocooler. The magnitude and the phase of the opposing forces are derived from a sensor signal. The interpretation of the sensor signal is difficult, since the spectral power density of the vibration of the cryocooler must be interpreted. This power density changes during the operation and depends among other things on the temperature of the cold head.
The problem of interpreting the sensor signal is solved in the document U.S. Pat. No. 6,131,394 in that a vibration sensor disposed on the measuring cell is part of a control loop, wherein the parameters of the transfer function are changed depending on the operating condition. The signal is then processed by a signal processor, which is necessary because the relationships between vibrations at the measuring cell or specimen have a complex relationship with the necessary driving force at the opposing mass. Although the proposed solution compensates the vibrations at the specimen, no solution can be deduced therefrom to reduce the vibrations by way of the attachment of the cryocooler.
Furthermore, it was proposed in the document DE 39 16 032 in the context of active noise cancellation to sequentially connect several control loops in a row for generating a counter-oscillation. However, this document also does not disclose any solution for the three nested damping systems that address conducting the cold to the specimen in a measuring cell and also ensure that vibrations are kept away from the instrument table and the microscope.